The invention relates to a method for the manufacture of bipolar transistor structures with self-aligned emitter-base regions for extreme high frequency circuits. Both the emitter region as well as the base region in the semiconductor substrate are generated by means of out-diffusion of the structures comprising doped polysilicon layers deposited directly on the substrate. The emitter region is centrally generated in the base region by means of silicon oxide layers as masking and insulating layers such that the active base zone arises under the emitter region, as does an inactive base zone which is symmetrical thereto. Dry etching methods producing vertical etching profiles are employed for structuring the silicon oxide and polysilicon layers.
In traditional bipolar structures, the base and emitter regions are generated by means of ion implantation of dopants and out-diffusion of the doped ions into the silicon substrate, whereby the emitter must be adjusted in overlap-free fashion into the base region with corresponding adjustment tolerances. This requirement causes relatively large structures which are also determined by the entire metal grid since contacts must be produced from the metal interconnect level both to the emitter and collector zone as well as to the base zone.
The self-adjusted manufacture of emitter and base, such as has already been proposed in the initially described method, fundamentally excludes these disadvantages. The realization of the structures with the assistance of doped polysilicon enables the exploitation of various further advantages such as significantly lower base resistances, smaller capacitances, and the possibility of a local wiring in the polysilicon levels.
A method in which self-adjusted emitter-base bipolar structures for high demands given low powers and low gate transit times can also be manufactured may be derived from an article by Ning et al in the IEEE Transactions on Electron Devices, Vol. ED-28, No. 9, September 1981, pages 1010 through 1013, incorporated herein by reference. Given this method, the separation of the emitter-base contact is defined only by the thickness of the edge oxide. This amounts to 0.4 .mu.m in the region of the silicon substrate. The edge oxide is deposited after the structuring of the boron-doped polysilicon layer forming the base terminal for the purpose of exposing the emitter region. This occurs partially by means of dry etching which produces vertical etching profiles and, for the rest of the layer thickness, by means of a doping-selective, wet-chemical etching in a hydrofluoric acid/nitric acid/acetic acid etching mixture.
The self-adjusted emitter-base structures obtained by means of the known methods have the deficiency that they cannot be manufactured with sufficient reproducibility, since the lateral underetching that is unavoidable given the wet-chemical etching step is difficult to control.